The present invention relates to a shift control system for a continuously variable traction roller transmission.
A shift control system for a continuously variable traction roller transmission is disclosed, for example, in JP 2-131945. This shift control system comprises a shift valve including a stepper motor having a rotational position determined in accordance with a commanded speed ratio, a sleeve axially movable by the stepper motor, and a spool slidably fitted in the sleeve. The spool is axially driven to correspond to an actual speed ratio via a cam, a link, etc. When the actual speed ratio corresponds to the speed ratio commanded to the stepper motor, the relative positional relationship between the sleeve and the spool falls in a normal state, supplying hydraulic pressure to a hydraulic cylinder apparatus so as to maintain the speed ratio at that moment. On the other hand, when the relative positional relationship between the sleeve and the spool is out of the normal state, hydraulic pressure is supplied to the hydraulic cylinder apparatus so that the actual speed ratio corresponds to the commanded speed ratio.
With the aforementioned shift control system for a continuously variable traction roller transmission, however, there is a disadvantage. The disadvantage is due to generation of friction resistance on both outer and inner peripheries of the sleeve when moving, thereby requiring a great operating force to displace the sleeve. Particularly, since the viscosity of hydraulic fluid increases when the temperature is extremely low, this disadvantage becomes striking at low temperature. Thus, the stepper motor for moving the sleeve must have great torque, resulting in an increased size of the stepper motor.
It is, therefore, an object of the present invention to provide a shift control system for a continuously variable traction roller transmission which can be operated by a stepper motor having small torque.